Which characteristic or component is shared by skeletal muscle and smooth muscle? (A) Thick and thin filaments arranged in sarcomeres (B) Troponin (C) Elevation of intracellular [Ca2+] for excitation–contraction coupling (D) Spontaneous depolarization of the membrane potential (E) High degree of electrical coupling between cells
The Correct Answer and Explanation is :
The correct answer is (C) Elevation of intracellular [Ca2+] for excitation–contraction coupling.
Explanation:
Both skeletal muscle and smooth muscle share the fundamental process of excitation-contraction coupling, where an increase in intracellular calcium ion concentration ([Ca²⁺]) triggers muscle contraction. However, the mechanisms involved in this process differ between the two muscle types.
- Skeletal Muscle:
- Skeletal muscle contraction is primarily controlled by the neuromuscular junction, where motor neurons release acetylcholine. This leads to an action potential in the muscle fiber, which travels along the sarcolemma and into the T-tubules.
- The action potential then triggers the release of calcium ions from the sarcoplasmic reticulum (SR), which binds to troponin. This binding leads to a conformational change that moves tropomyosin, exposing binding sites on the actin filaments and allowing cross-bridge formation between actin and myosin, resulting in muscle contraction.
- Smooth Muscle:
- Smooth muscle contraction is regulated by intracellular calcium levels, but it doesn’t rely on a neuromuscular junction or troponin for its contraction. Instead, calcium binds to calmodulin, which activates myosin light-chain kinase (MLCK). This enzyme phosphorylates the myosin light chain, allowing myosin to interact with actin, thus facilitating contraction.
- Calcium in smooth muscle can enter the cell through voltage-gated channels or be released from the sarcoplasmic reticulum.
Although the exact mechanisms differ, both skeletal and smooth muscle rely on the elevation of intracellular calcium to initiate the contraction process, which is a shared characteristic.
Other Options:
- (A) Thick and thin filaments arranged in sarcomeres: This is specific to skeletal and cardiac muscle. Smooth muscle does not have sarcomeres.
- (B) Troponin: Troponin is specific to skeletal and cardiac muscles and is not found in smooth muscle.
- (D) Spontaneous depolarization of the membrane potential: This is more characteristic of smooth muscle, especially in the case of certain smooth muscles like those in the gastrointestinal tract. Skeletal muscle does not typically exhibit spontaneous depolarization.
- (E) High degree of electrical coupling between cells: This is more characteristic of smooth muscle, especially in multi-unit and single-unit smooth muscles, where gap junctions provide electrical coupling between cells. It is not a typical feature of skeletal muscle, which has discrete motor units.